1. Field of the Invention
The present invention generally relates to an apparatus and method useful for measuring the viscosity of a fluid. The present invention is particularly useful for measuring the viscosity of a liquid in an environment which is corrosive or harmful to typical ball or roller bearings. More particularly, the present invention relates to an apparatus and method employing flexural or torsional bearings and useful for making viscosity measurements in environments harmful to typical ball or roller bearings.
2. Description of the Background
It is desirable to determine the viscosity of fluids, particularly liquids, under a variety of conditions of temperature, pressure, atmosphere and the like to simulate the expected working environment in which the fluid will be employed. The liquids to be tested and/or the environment in which the tests are performed may be corrosive or at least harmful or deleterious to the testing equipment. For example, it is desirable to determine the viscosity of drilling fluids under conditions simulating the actual borehole conditions to which the fluids will be subjected. It is not uncommon to encounter temperatures in excess of 400.degree. F. and pressures in excess of 10,000 psi in deep well bore operations. Accordingly, in order to properly evaluate drilling fluids useful in deep drilling operations, it is desirable to obtain viscosity measurements conducted under these harsh conditions. In fact, it is desirable to conduct these tests under even more extreme conditions, e.g., at temperatures in excess of 700.degree. F. and at pressures in excess of 20,000 psi. Further, it is often desirable to perform these tests in the presence of corrosive fluids, e.g., hydrogen sulfide, which are often encountered in the drilling operation.
Apparatus for measuring the viscosity of liquids includes conventional rheometers. These conventional rheometers generally comprise a cylindrical bob suspended within a concentric tubular sleeve for immersion in the fluid to be tested. These devices further comprise a means for rotating the sleeve at a known velocity. Finally, these devices include a means for measuring the drag transferred to the suspended bob from the fluid in the annulus between the rotating sleeve and suspended bob. The bob and sleeve are suspended from a stationary frame, the bob being suspended by conventional ball or roller bearings. The condition and lubrication of these bearings is extremely important to the proper functioning of conventional rheometers. It will be appreciated by those skilled in the art that the lubrication and condition of the suspension and bearing system is highly critical to precise and accurate measurement of the angular motion or torque imparted to the bob. These bearings suffer from many disadvantages, particularly when used in remote, corrosive or harmful environments. Conventional ball or roller bearings tend over time to become pitted or gummed up, resulting in inaccurate viscosity measurements and, eventually resulting in failure of the instrument. These problems are accelerated and accentuated in rheometers used to make accurate and precise viscosity measurements in remote, corrosive or harmful environments.
Accordingly, there has been a long felt but unfulfilled need within the testing industry for a rheometer-type apparatus and method for measuring the viscosity of a liquid without relying upon ball or roller bearings to facilitate the measured motion.